1. Field of the Invention
The present invention relates to a driving force transmitting apparatus for a four-wheel drive vehicle based on a front engine and front drive system.
2. Description of Related Art
As a driving force transmitting apparatus for a four-wheel drive vehicle, there is a known technique described in Japanese Patent Application Laid-open No. 63-162334 for example. This technique (which will be referred to as conventional art 1, hereinafter) is applied to a four-wheel drive vehicle based on an FF (front engine and front drive) system in which the engine is disposed horizontally as shown in FIG. 1. Reference number 2 denotes an engine, symbols Tfl and Tfr denotes left and right front wheels, respectively, symbols Trl and Trr denote left and right rear wheels, respectively, and reference number 4 denotes a driving force transmitting system for varying a driving force distributing ratio to each of the wheels Tfl, Tfr, Trl and Trr.
The driving force transmitting system 4 includes an automatic transmission 6 for gear shifting a driving force from the engine 2 at a predetermined gear shift ratio, and a transfer 8 for dividing the driving force from this automatic transmission 6 to the front wheels Tfl and Tfr as a front wheel driving torque, and to the rear wheels Trl and Trr as a rear wheel driving torque. The front wheel driving torque is transmitted to the front wheels Tfl and Tfr as a driving force through a front wheel drive shaft 10, and the rear driving torque is transmitted to the rear wheels Trl and Trr as a driving force through a propeller shaft 12 and a rear differential device 32. The transfer 8 includes a center differential device 16 to which the driving torque is transmitted from an output gear 6a of the automatic transmission 6, a front differential device 18 connected to the center differential device 16 for transmitting the front wheel driving force to the front wheels Tfl and Tfr, a differential control clutch 20, a control device 22 and various input systems 24.
The control device 22 sends a pulse signal of a predetermined duty ratio in accordance with information sent from the various input systems 24, thereby operating the differential clutch 20. When a grip force of magnitude in accordance with the duty ratio is generated between a differential case 16a and a rear wheel output side gear 16b, a differential generated between a front wheel output side gear 16c and a rear wheel output side gear 16b is suppressed in accordance with the grip force, so that a torque is transmitted from the wheels rotating faster to the wheels rotating slower.
Reference number 30 denotes a transferring gear which is to be connected to the rear output side gear 16b, and reference number 32a denotes a rear differential gear of a rear differential device 32. An input gear 12a formed at a front end of the propeller shaft 12 meshes with the transferring gear 30, and an output gear 12b formed at a rear end of the propeller shaft 12 meshes with the rear differential gear 32a. The number of teeth of the output gear 12b is 12, and the number of teeth of the rear differential gear 32a is 41. With these number of teeth, the front wheel output side gear 16c which is at the side of the front wheels of the center differential device 16 always rotates 2.4% faster than the rear wheel output side gear 16b which is at the side of the rear wheels.
With this arrangement, since relative rotation (differential) between the front wheel side and the rear wheel side of the center differential device 16 is generated when the vehicle is running, it is possible to limit the torque amount transmitted from the wheels rotating faster to the wheels rotating slower depending upon a degree of grip force for suppressing this differential by the differential control clutch 20. More specifically, when the differential generated between the front and rear wheels is suppressed with a weak force, only a torque in an amount corresponding to this weak force can be transmitted from the wheels rotating faster to the wheels rotating slower. On the contrary, when the differential is suppressed with a strong force, a large amount of torque can be transmitted from the wheels rotating faster to the wheels rotating slower. Therefore, a torque distribution ratio between the front and rear wheels can arbitrarily be set.
Further, as a four-wheel drive vehicle of FF system in which electric control means or hydraulic control means is not used, there is a known apparatus comprising a propeller shaft for transmitting a driving force from a transfer disposed at the side of front wheel to a rear differential device disposed at the side of rear wheels, and viscous couplings as a limit slip differential connected to the propeller shaft in series (this known technique is referred to as conventional art 2, hereinafter).
In the conventional art 1 shown in FIG. 1, the input gear 12a of the propeller shaft 12 meshed with the transferring gear 30 and the input gear 12b of the propeller shaft meshed with the rear differential gear 32a have the same number of teeth (12). Therefore, if the propeller shaft 12 rotates, since the input gear 12a and the output gear 12b are meshed with each other in the same teeth meshing state, gear noise increases in many cases.
In the conventional art 2 in which the viscous couplings are connected to the propeller shaft in series, a first gear meshed with a gear at the side of the transfer of the propeller shaft and a second gear meshed with a gear at the side of the rear differential device also have the same number of teeth. Therefore, if the viscous couplings are operated to such a degree that an inner plate and an outer plate are relatively rotated, a difference in the number of rotation between the first and second gears is slightly varied, a noise of the first gear and a noise of the second gear are superposed such as a surge phenomenon, and beat sound may be generated in many cases.
Further, in the conventional art 1, a gear ratio between the transferring gear 30 (the number of teeth: 42) at the side of the transfer 8 and the input gear 12a (the number of teeth: 12) is set to 3.500 (42/12), a gear ratio between the rear differential gear 32a (the number of teeth: 41) at the side of the rear differential device 32 and the output gear 12b (the number of teeth: 12) is set to 3.417 (41/12), and a difference between the gear ratios at the side of the transfer 8 and at the side of the rear differential device 32 is set large (3.500-3.417=0.083). If the gear ratio difference is set large, parts in the center differential device 16 are rotated always in slipping state when the vehicle is running, and the temperature of the lubricant oil charged in the apparatus is prone to rise, which exerts an influence on the durability of the center differential device 16.